The invention relates to a method for determining the volumetric proportion of liquid water and the density of snow and to a device for carrying out the method.
The determination and the effects of the global climatic changes with regard to ice and snow requires the measurement of the snow conditions, particularly its density and the liquid water content thereof.
These are also the most important values for issuing avalanche and flood warnings.
The expected filling degrees of reservoirs of northern hydro-electric generating stations must also be predicted far in advance using these parameters.
M. Schneebeli et al., of the Eidg. Institut fur Schnee and Lawinenforschung SLF, Davos/Switzerland, and Meteo France, St. Martin d' Heres/France, report a possible solution in the publication "Measurement of Density and Wetness in Snow Using Time-Domain-Reflectometry" in print by the publication Am. Glacial, 26, 1997. The article also describes other solutions with short critical comments. Their solution is based on calibration in a laboratory. They were only able to determine the density of dry snow in a sufficiently good way. It is pointed out that, for the determination of two unknowns (density and moisture), at least two independent information sources (linearly independent measurement results) are required. Since snow is a three-component mixture, three sources are required wherein the third source is provided by the law of mass conservation. The above authors have utilized only one source. Therefore, measurements in moist snow had to fail as reported by them.
The report by Sihvola, A. and Tiuri, M. 1986, Snow fork for field determination of the density and wetness profiles of a snow pack, IEEE Trans. Geosci. Remote Sens. GE 24(5), 717-721, describes a snow fork. This apparatus measures at about 1 GHz the complex (that is, two sources) dielectricity coefficients, wherein at the same time the density and moisture content of the snow is determined. However, a continuous measuring operation of this apparatus was not reported. Because the imaginary part of the DK is extremely small at these frequencies, the measurements are highly uncertain particularly if the snow is contaminated. Furthermore, the apparatus measures only over a very small space.
Another solution by A. Denoth: The monopole antenna: A practical Snow and Soil Wetness Sensor, in IEEE: Transactions on Geoscience and Remote Sensing. Vol. 35, No. 5, September 1997 interconnects the KD-value with the density if the snow is dry (on the basis of empirical knowledge). The density change over time is extrapolated for a later time and is then taken as a given value.
With the density and the measured KD value of the moist snow mixture, the moisture content is then determined. This procedure is inaccurate since the extrapolation presumes a certain snow development. If, however, in the mean time, there was a snow melt or re-freezing, that presumption is incorrect. The method fails completely if the time during with the snow is dry is too short, or if the falling snow is already moist.
None of the known apparatuses can eliminate the falsifying effects of the gap between the snow and the instrument, which is always present. Also for this reason, there is no apparatus with an expansive or large measuring area. No apparatus could indicate local inhomogeneities along a path or over an area.
It is the object of the invention to provide a method of the type described above in such a way that disturbances by the measuring system itself are not possible and an apparatus for carrying out the method.